1. Field of the Invention
The present invention relates to a non-contact type rotation-angle sensing device that detects a rotation angle of a rotating object to be measured with the use of a permanent magnet and a rotation angle sensor having a magnetoresistance element.
2. Description of the Related Art
Under the background of improvement in durability and higher resolution in detection accuracy, it is a recent trend that rotation-angle sensing devices such as a device for detecting a throttle valve opening of an intake control system of an internal combustion engine have been changed to a non-contact type. Actually various types of non-contact type rotation-angle sensing device have been proposed. It is publicly known that a Hall element or a magnetoresistance element (MR element) is used in non-contact type rotation-angle sensing devices.
In a conventional non-contact type rotation-angle sensing device disclosed in the Japanese Patent Publication (unexamined) No. 1994-93921, plural rotation angle sensors (nonrotatable elements) are arranged facing to permanent magnets (rotatable elements) disposed opposite to each other. In this known arrangement, although a rotation angle sensor output is outputted nonlinearly in case of using only one rotation angle sensor (Hall element), using plural rotation angle sensors makes it possible to secure linearity in such a manner as judging a rotation angle sensor that outputs linearly at a specific angle, thereby switching the output. However, in order to judge the switching and detect the rotation angle, plural rotation angle sensors are required, which results in a problem of increase in cost.
In another conventional non-contact type rotation-angle sensing device disclosed in the Japanese Patent Publication (unexamined) No. 1997-72706, a soft magnetic material (nonrotatable element) having a rotation angle sensor (Hall element) is arranged at the center of permanent magnets (rotatable elements) disposed opposite to each other. It is, however, difficult to stabilize density of magnetic flux passing through the rotation angle sensor through the soft magnetic material when the permanent magnets rotate, and it is not possible to secure linearity. In other words, a problem exists in that the distance between the permanent magnets and the soft magnetic material has no linearity with respect to the rotation angle. Moreover, when the rotating direction is reversed thereby reversing the magnetic flux passing through the soft magnetic material, magnetic hysteresis may cause further deterioration in linearity, which is a further problem.
The Japanese Patent Publication (unexamined) No. 2001-317909 discloses a structure of a conventional non-contact type rotation-angle sensing device. In this conventional sensing device, to secure linearity of density of magnetic flux passing through a rotation angle sensor (Hall element) with respect to the rotation angle, a magnetic circuit is formed in permanent magnets with the use of a rotor core member and a stator core is disposed around the rotation angle sensor. Thus, it is possible to secure the linearity of the density of magnetic flux flowing into the stator core owing to the rotation angle. However, this known device also has a problem of linearity being affected by magnetic hysteresis when the rotating direction is reversed thereby reversing the direction of the passing magnetic flux.
The Japanese Patent Publication (unexamined) No. 1999-94512 discloses a constitution of a further conventional non-contact type rotation-angle sensing device in which a magnetoresistance element (MR element) is used. This magnetoresistance element detects direction of the magnetic flux, thus making it possible to detect the direction of the magnetic flux generated from a permanent magnet (a flux from the N-pole to the S-pole). It is further possible to detect a magnetic flux density on condition that at least the minimum magnetic flux density required for the detection (for example, approximately 0.01 to 0.03 T in case of using NiFe (Permalloy)) is used. This device does not require any magnetic circuit complicated in constitution such as Hall element. Furthermore, it is not necessary to arrange any stator core or magnetic material around the magnetoresistance element. As a result, it is neither necessary to make any magnetic flux flow into the rotation angle sensor (magnetoresistance element) through a stator core or soft magnetic material, nor necessary to cope with the problem of the magnetic hysteresis generated at the time of reversing the rotating direction. However, the most part of the magnetic flux generated from the permanent magnet leaks outside the rotation angle sensor (the magnetoresistance element), and therefore it is essential to use a large size permanent magnet, which results in increase in cost. The permanent magnet is expensive because it is essential to use rare earth metals (such as Sm—Co system and Nd—Fe—B system) of superior heat-resisting property and coercive force as the material of the permanent magnet.
In the case where any non-contact type rotation-angle sensing device is incorporated in an intake control system, a control unit computes necessary engine speed, output, etc. on the basis of rotation angle sensor output. If the rotation angle sensor output is erroneously provided, the engine control becomes unstable, whereby unstable idle rotation, poor fuel efficiency, etc. will occur. In the worst case, there is a likelihood of engine stopping or running out of control. In order to cope with these problems, two rotation angle sensors are used in view of fail-safe operation and, moreover, the non-contact type rotation-angle sensing device itself is undesirably large-sizes eventually resulting in increase in cost.
As discussed above, in any of the conventional non-contact type rotation-angle sensing devices in which a Hall element is used to detect magnetic flux density, it is necessary to arrange a complicated magnetic circuit from the viewpoint of securing the linearity of the change in density of the magnetic flux passing through the sensor with respect to the change in rotation angle. As a result, there remains a problem of magnetic hysteresis generated by the change in magnetic flux due to rotation. In any of the conventional non-contact type rotation-angle sensing device in which a magnetoresistance element is used, it is essential to detect direction of the magnetic flux, and it is possible to detect the angle using the permanent magnet. A problem, however, exists in that it is necessary to use a large size permanent magnet to secure the linearity.